This invention relates to iron powders and methods of producing iron powders, and specifically to methods of producing iron powder from iron oxide powder.
For many centuries iron products have been made by heating iron oxide in the presence of carbon, thereby reducing the iron oxide it to pure iron in a molten state along with a quantity of waste slag. The molten iron is separated from the waste slag and either cast into billets or poured into product molds. In order for this process route to be used commercially large and very expensive equipment must be used. Recently however iron products have been manufactured by two methods commonly referred to as powder metallurgy (PM) and metal injection molding (MIM).
In powder metallurgy, iron powder in combination with a small amount of binder is positioned within a mold and compressed by a hydraulic press to form a blank which is then sintered to form the finished product. Products produced by powder metallurgy are of relatively simple configuration as the molds used to produce the blanks are limited in their ability to produce complicated shapes.
In metal injection molding, an extremely pure and extremely fine iron powder in combination with a binder, such as wax-polypropylene, is injected into a product mold under pressure to compress the combination within the mold to form a blank. The blank is then removed from the mold and heated causing the binder to melt out and the remaining iron powder to bind together to form the finished product, i.e. the blank is sintered. This method of producing finished goods has been proven to be safer, more economical and easier in producing small and intricate finished goods than methods of production using molten iron. However, this method must use iron powder of a smaller and more consistent spherical configuration than with powder metallurgy.
Iron powder used in the just described metal injection molding (MIM) method typically has a median particle size diameter of less than 20 microns. In the past iron powder for MIM use has been produced by two methods. One such method of production has been by a chemical process wherein extremely small iron oxide spheres are produced by chemical vapor decomposition. This method produces an iron powder product commonly referred to as carbonyl iron powder. The capitol and operating cost associated with this method results in the finished iron powder being economically limited.
Accordingly, it is seen that a need remains for a method of producing iron powder in a more economic manner. It is to the provision of such therefore that the present invention is primarily directed.
In a preferred form of the invention a method of producing iron powder used in metal injection molding comprises the steps of heating a supply of iron oxide powder having a median particle size of less than 1000 microns in a reducing agent atmosphere to a temperature between 1000xc2x0 F. and 2100xc2x0 F., thereby reducing the iron oxide powder to iron powder. The iron powder is then cooled in an inert gas atmosphere to a temperature below 150xc2x0 F. and milled in an inert gas atmosphere to a median particle size diameter of less than or equal to 20 microns.